Molded plastic parts manufactured by injection molding have widely been used as interior and exterior parts for automobiles, housings of electrical appliances etc., articles for daily use, and the like because they are light in weight, easily molded into various shapes, and low in cost. In particular, when being used as a covering member or a decorative member for an article surface, such as a cover or a garnish, the molded plastic part not only can easily fit with the shape of the installation portion but at the same time can also be light in weight and manufactured cheaply. Such a resin-made cover or garnish is generally formed into a thin-wall shell construction, and is integrally formed with a reinforcing structure such as a rib, a mounting structure for mounting in a mounting portion, a joint structure for joining the molded components to each other, and the like on the back surface of a design surface (hereinafter referred to as the design back surface) disposed on the outer surface side of an article. In the portion in which these structure parts (hereinafter referred inclusively to as a reinforcing structure part) are formed, the fill of resin is greater than that in other portions, so that a molding defect such as a weld line or a dent is liable to occur on the design surface.
To solve the above problem, JP5-53916U has disclosed measures such that the reinforcing structure part is disposed on the back surface of a portion concealed after mounting, avoiding a portion conspicuous in appearance, so that even if a dent occurs on the design surface side, the appearance is not affected, and measures such that in the portion conspicuous in appearance, the thickness of a connecting part connecting with the reinforcing structure part is made small to restrain a dent. However, it is difficult for these measures to be taken for a part in which the whole of design surface is arranged at a conspicuous position, and additionally, if the thickness of the connecting part is made small, a decrease in strength caused by stress concentration is inevitable. Therefore, these measures are not effective or universal.
To improve the texture of a molded plastic part, the molded plastic part may be painted after molding. However, in recent years, from the viewpoint of environmental protection, in a situation where a demand for reducing environmental pollutants emitted in the manufacturing process increases, there is the risk that volatile organic compounds contained in general paints are released into the atmosphere and adversely impact the environment. Accordingly, the use of a solvent in place of the volatile organic compound, the reduction in quantity consumed of paint itself, and the like have been considered as measures thereagainst.
On the other hand, a method being increasingly used obtains a molded plastic part providing satisfactory texture without painting by performing injection molding using a colored resin that has actually been colored in advance by a coloring matter such as a pigment, a dye, or the like. Advantages of unpainted molded plastic parts manufactured by using a colored resin include the fact that no volatile organic compounds are generated and that costs can be reduced by the omission of the painting process. Another advantage is that the texture can be maintained for many years because the paint film does not peel off.
Also, a method has been used in which to enhance the texture of unpainted molded plastic part manufactured by using the colored resin, a bright material such as aluminum flakes and glass flakes is added to the colored resin, by which a metallic or bright texture is given to the molded plastic part. However, an injection molding process using a bright material has the problem that the flow state of the resin in the mold at the time of molding tends to appear on the design surface after molding. Therefore, if the flow of resin at the time of injection molding is uneven, a portion having a different color tone is produced, which is recognized as an appearance defect.
Examples of such an appearance defect occurring on the design surface due to an uneven resin flow include a weld line and a flow mark. Although these defects also occur in the case of general resin materials other than colored resins, they especially appear on injection molded components manufactured by using the colored resin, and the number of cases recognized as an appearance defect increases. Therefore, when an unpainted molded plastic part manufactured by using a colored resin is used as a product, it is necessary to more effectively restrain the occurrence of these defects.
Next, the causes for the occurrence of the above-described molding defects are described in detail by giving a specific example.
FIGS. 6 to 8 show a shift lever knob garnish 51 mounted on an automotive shift lever 501, which is molded by a conventional general method. FIG. 6A shows typical weld lines Wp and Wq and a typical flow mark M occurring on the design surface when the shift lever knob garnish 51 is injection molded.
Generally, since a gate through which molten resin is poured into the cavity of an injection molding die leaves its cut traces after molding, the gate is set at a location as inconspicuous as possible. Also, a reinforcing rib etc. provided on the design back surface of a molded part body constituting the design surface is generally made thinner than the body. In the example shown in FIGS. 6 to 8, a gate 58 is set on a design back surface 52b close to the end edge so that resin can be filled surely in a molded part body 52 to allow the resin to flow into the rib (a reinforcing part 53) from the body part 52 side.
In this case, however, the flow mark M is liable to occur on the design surface on the side opposite to the gate of the molded part body 52. This flow mark M is caused by a phenomenon that the flow of resin arriving at the inner surface corresponding to a design surface 52a of the interior of the die is in a jet state. In the case of a gate with a set up boss, described later, as shown in FIG. 9, even at a level that is not recognized as an appearance defect for the ordinary molded component, a difference in color tone is recognized in the injection molding using a bright material.
A flow mark does not occur if the gate through which resin is poured is provided in the direction parallel with the design surface, such as a side gate or a film gate. In this case, however, after the gate has been separated from the molded component, the cut surface is conspicuous, so that in many molded plastic parts, this type of gate cannot be used from the viewpoint of product appearance. Therefore, in the case of a molded plastic part in which the outer periphery of design surface is also visible to the passengers, such as the shift lever knob garnish, it is necessary to pour resin from the back surface side of the molded part body as described above, meaning that measures against the flow mark become indispensable.
Further, the reinforcing part 53 projectingly provided on the design back surface 52b of the body part 52 is made up of ribs 56p and 56q and a rib 57 that connects these ribs 56p and 56q to each other in the width direction in the end portion close to the gate 58, and all of these three ribs are provided so as to be erected from the design back surface 52b. On the design surface 52a of the molded part body 52 at the location where these ribs 56p, 56q and 57 are present, the tip end shape of resin flow sometimes exhibits unevenness having a concave shape locally, which may cause a weld line.
FIG. 9 shows images obtained in the case where short shots are performed intentionally to check the actual injection molding process of the shift lever knob garnish 51, and molded components of four stages in which the filling of resin is not finished are injection molded. From the left-hand side images, the forming process of the design surface can be observed, and from the right-hand side images, the forming process of the body part 52 and the reinforcing part 53 can be observed. Images (a) show the forming states immediately after the start of injection molding, images (d) show the forming states immediately before the finish, and images (b) and (c) show transient forming states between the forming states shown in images (a) and (d). It is found that in the forming states shown in images (c) and (d) of these four forming states, the tip end shape of resin flow is concave locally, and therefore the weld lines Wp and Wq shown in FIG. 6A occur.
As described above, the weld line occurring when the molded plastic part is formed by injection molding the colored resin is caused by the phenomenon that the resin flow is delayed locally in the forming process, and therefore the tip end shape of flow becomes concave locally. To solve this problem, a molded plastic part is required which has a shape and construction such that the flow of resin is even as a whole, the occurrence of a local concave shape at the tip end of flow is prevented, the occurrence of a weld line can be restrained, and a problem of insufficient rigidity or the like is not caused.